The United States Environmental Protection Agency (EPA) recognizes 14 metals as air pollutants when emitted in exhaust emissions from sources such as the stacks of industrial incinerators, furnaces, and boilers. Conventionally, these sources are monitored for compliance with EPA regulations through a series of manual test methods. These methods require extraction of large volumes of exhaust gases from an exhaust stream over a period of one to three hours. The targeted emissions, e.g., metal aerosols, vapors, and particulates are collected in filters and typically are analyzed offsite. Recent technology now provides the capability to analyze emissions nearly continuously via robust in-stack sensors connected to onsite monitors. See U.S. Pat. No. 5,596,405, Method and Apparatus for the Continuous Emissions Monitoring of Toxic Airborne Metals, issued to Seltzer et al, Jan. 21, 1997. Historically, as the technology becomes available, EPA modifies regulations to take advantage of the improved capability. In this case, the regulations are re-written to include compliance criteria based on availability of "continuous emission" monitors that can readily provide emissions criteria over both short (e.g., one hour) and long (e.g., 24 hours) time intervals. Further, the new robust sensor/onsite monitor provides the inherent capability to time-resolve measurements and assure interim compliance in real time, heretofore unavailable using manual methods or low cost automated methods.
The majority of exhaust gas pollution emission analyzers use the detected species in the gaseous state. Among these are analyzers for detecting carbon monoxide (CO), nitrogen oxides (NO.sub.x), and sulfur oxides (SO.sub.x). Commercially prepared and certified gas mixtures are available as aerosols for use in evaluating emission analyzers. The same gas source can be introduced into the candidate analyzer and the reference analyzer, permitting a side-by-side comparison. Similarly, a specific gas mixture can be inserted into an exhaust gas stream to permit comparative measurements of in-stack sensors/monitors using both a candidate test method, e.g., a preferred embodiment of the present invention, and a reference test method, e.g., an EPA-approved manual method.
A significant factor in achieving EPA acceptance of the new generation of "in-stack" sensor/monitors is the ability to test them in the same "real time" that they are designed to operate. Further, the chosen test method should be efficient, accurate, and reliable for a wide range of exhaust streams and operational environments. Specific requirements include the ability to compare performance of the new monitoring technology to the EPA-approved reference methods for determining compliance, i.e., manually derived testing. One of the most basic problems to overcome in this comparison is providing representative exhaust streams composed of a known and relatively constant multi-element (metal) constituent for a given time period. Consider that the constituent need be both temporally consistent, i.e., be held constant, and offer a wide range of representative metals, including weight percentage levels, in the exhaust stream. That is, the concentrations of the various metals and the timeline for insertion in the exhaust stream should be known a priori and able to be controlled accurately over time.
Rarely does an unmodified exhaust stream exhibit metal emissions of the necessary elemental diversity and compositional and temporal stability to enable efficient, yet accurate and reliable, comparative testing. Metal emissions within a typical actual exhaust stream are sporadic, short-lived, and limited in elemental composition by the specific fuel or waste feed used as input. For example, inserting enough metal in the original fuel of a combustor (as metal oxides or salts, for example) to achieve emissions levels necessary to test the competing emissions sensors/monitors weld most likely violate the EPA's regulations for control of hazardous pollutants! Also, because the combustor is equipped with scrubbers and other emissions control devices to prevent excess emissions, providing enough excess metals at the input, i.e., in the fuel, may not be possible to attain the required levels for testing the sensors/monitors at the output, i.e., the exhaust stack.
Another method tried with little success is the insertion into the exhaust stream of metals via nebulization, i.e., spraying an aqueous metal solution. The theory is that given the high heat of the exhaust stream there will be sufficient latent heat to evaporate the water vapor in the nebulized metal solution, leaving a dry aerosol with entrained metals. However, experience with such methods has shown that in typical exhaust streams, the gases lack capacity to absorb additional moisture. This results in incomplete evaporation and water droplets containing entrained metals transit the exhaust stack. These droplets are deposited on the hardware used for manual extraction where they then quickly evaporate on the hot surface of the hardware and deposit metal for subsequent analysis. Thus, there is a dramatic difference between the results obtained with the candidate in-stack sensor/monitor and the EPA-approved manual method. The manual method can recover the evaporated metals on the hardware surface since the extraction hardware is washed and the metals recovered. No such provision is available for the "real time" in stack sensor/monitor.
Yet another approach is the generation and insertion of organic vapors with entrained metals. This is accomplished by chemically reacting two substances intentionally inserted and brought into close contact in the exhaust stream. A major disadvantage of this method is the toxicity of the substances needed to carry out the reaction. Further, even considering the handling difficulties of candidate substances, this method still does not provide the necessary aerosol needed to insure a valid test comparison. A viable solution need provide a source of:
dry, multi-element aerosol with entrained metals of interest, PA1 dry aerosol-entrained metals independent of fuel or waste feed, and PA1 dry aerosol-entrained metals independent of temperature and moisture content, that ideally is compact, lightweight, easy to use, reliable, and provides a reproducible output. PA1 a combustion chamber PA1 a fuel tank, PA1 a container for aqueous-entrained hazardous elements, e.g., metal salts PA1 a forced-air draft fan, PA1 a pump, PA1 a nebulizer, PA1 an air compressor, and PA1 metal ducting. PA1 simplified test systems using COTS hardware; PA1 use of reconfigurable pumps; PA1 simplified design of alternate configurations; PA1 inexpensive fabrication; PA1 reduced man-hours for operation; PA1 reduced system complexity; PA1 reduced system capital costs; PA1 improved test robustness; PA1 low maintenance costs; PA1 increased flexibility in test conduct; PA1 fewer tests or higher duty factor per test or both; PA1 high reliability; and PA1 ready upgradability.
Certified sources of metal air pollutants, similar to the commercially prepared gas mixtures noted above, are not presently available. Actual exhaust streams having entrained metals are primarily aerosols and particulates. Rarely do they consist of vapors. It is not practical, assuming physical possibility, to commercially prepare a homogenous mixture of targeted species (i.e., EPA-defined hazardous metals), contain it in a pressurized bottle, and be able to insert amounts of this mixture on a reproducible basis into a "front-end" of a sensor/monitor.
A solution to this testing problem is a system and method for introducing a dry gas mixture of known metal composition into the exhaust stream at known times and for known time intervals. It is not even critical at this juncture that the concentration of the gas/metal mixture be precisely known at input. So long as the mixture is inserted at a constant rate in the exhaust stream for consequent measurement using the manual EPA-approved method and the sensor/monitor to be tested, the system and method provide an efficient, reliable and accurate, solution. Insertion of the surrogate mix at or near the input end of the exhaust stream insures a homogenous mixture of existing exhaust gases and the surrogate mix by the time the exhaust stream reaches the sensor/monitor positioned near the output of the exhaust stream. Thus, a reliable alternate means for providing the necessary variety and levels of hazardous element emissions at the sensor/monitor, at a relatively constant level held relatively constant over a given time period, is provided as a preferred embodiment of the present invention.